Typical video and computer graphics display systems use a cathode ray tube (CRT) to convert electrical video signals into a luminous image. In its most basic form, the CRT is an analog device which includes an electron gun designed to emit a stream of electrons towards a phosphorous screen. The stream of electrons passes between two orthogonal pairs of charge plates before impinging on the screen. The two sets of plates are used to create an electric field which deflects the stream of electrons such that the stream of electrons impinges on the screen at a selected location. In a typical monochrome raster-scanned system, a brightness signal is applied to the gun of the CRT to control the intensity of the electron stream, and timing signals are applied to the two sets of plates to sweep the point where the stream of electrons impacts the screen across the screen row-by-row from top to bottom.
A typical CRT-based television system displays the image data as a serial stream of pixels, just as it is received. Vertical and horizontal synchronization signals, which are encoded along with the image data, are used to vary the charge on the CRT plates, thereby directing the electron stream to the proper point on the CRT screen. Because the synchronization signals encoded in the video signal control the aim of the CRT electron gun in real-time, the CRT-based display easily adapts to input video signals with differing frame rates. For example, CRTs used for computer displays must operate at a variety of frame rates such as 60, 66, 72, and 75 Hz, while PAL or SECAM televisions operate at 50 Hz. Additionally, a video source may drift about its specified frame rate. For example, if a videotape is stretched, the frame rate of the video signal will decrease. However, the video timing signals encoded in the video signal will alter the charge on the plates at the slower frame rate and the CRT-based display system will automatically adapt to the slower frame rate.